The use of renewable energy sources has become of paramount importance to develop our planet in a sustainable way.
Among the renewable sources, the recovery of energy radiated from the sun in order to produce electricity founds today more and more applications which can be broadly divided into photovoltaic applications and thermal applications.
Among the thermal applications of the solar energy the development of concentration solar power (CSP) plants is one of the field of technology development. The CSP plants concentrate the energy radiated from the sun by means of mirrors in order to heat a fluid at a temperature suitable to produce, in an heat exchange device, steam which in turn is used to produce electric power through a turbine and generator system.
Since the sun radiation is available during daylight only, different systems have been developed in order to store the energy recovered from the sun and use it during night time in order to ensure a wider timing of power availability according with the electric grid requirements.
In the state of the art the most common method to store the captured sun radiation energy is to heat a mass of molten salts, mostly a mixture of nitrates, during the day, while using these hot molten salts, either directly or indirectly, for producing steam and therefrom electric power.
The CSP plants use two different schemes to exploit the energy recovered from the sun radiation: in the first scheme (FIG. 10) the solar energy is transferred by heating an intermediate fluid, named Heat Transfer Fluid (HTF), generally a mixture of hydrocarbons, which is used during the day both to produce steam and to heat the stored molten salts mass increasing its temperature; in the second scheme (FIG. 11) the solar energy is directly transferred to the molten salts mass which is in part used to produce steam and in part used to store heat to be used at night time.
The heat storage system used to store the molten salts usually comprises one or more couples of tanks (named hot and cold salt storage tanks).
During molten salts heating, the same are transferred from the cold tank to the hot one, while, when the stored energy is recovered, the molten salts flow from the hot tank to the cold tank.
According to the different CSP plants schemes the cold tank operates within a temperature range varying from 270° C. to 440° C., while the hot tank temperature range varies from 350° C. to 560° C., with a temperature difference between the two tanks in the range of 70° C. to 200° C.
As an alternative to the two-tank storage system, thermocline storage systems can be used. A thermocline storage system is a single-tank system containing both the hot and cold fluid. This type of system relies on thermal buoyancy to maintain thermal stratification and discrete hot and cold thermal regions inside the tank.
CSP plants are today sized to produce electricity with an electric power output which ranges from 10 MW to 500 MW. Since the efficiency of the power generation system ranges from 30% to 50% and it is requested to produce electricity for 6 or 12 hours when the sun is not available, CPS plants need to store an amount of thermal energy in the range of 100 MWh to 20,000 MWh.
A typical thermal storage system in CSP plants being built today is in the range of 1,500 MWh.
In the case of a two-tank thermal storage system, assuming a temperature difference of 100° C. between the cold and hot tanks and considering the specific heat of typical nitrates mixture used as heat storage medium, it follows that a storage of more than 35,000 tons of molten salts is required.
Accordingly, the storage tank diameter is in the range of 20 to 70 meters while the tank height is in the range of 7 to 18 meters.
The molten salts storage tanks must be designed in order to avoid any possible leak of molten salts. This is achieved, inter alia, by avoiding any kind of connections, nozzles or the like below the maximum level which can be reached by the molten salts mass inside the tank. Accordingly, all connections are arranged on the upper part of the tank, preferably on its roof.
Moreover, the tanks need to be designed with their bottom being flat in order to avoid undesired stress concentrations.
Since no connections are present below the maximum molten salts mass level, the molten salts are generally transferred, either when heating or cooling, by means of aspiration pumps, preferably vertical centrifugal pumps, having the impeller arranged near the bottom of the tank and a shaft of suitable length protruding from the top of the tank.
In order to maintain a stable operation and to avoid cavitation phenomena, aspiration pumps, particularly centrifugal pumps, require a minimum positive suction head, that is a minimum liquid level at the pump suction side, namely over the pump impeller.
Accordingly, a minimum level of molten salts must always be present in the tank for properly operating the pumps. This results in the unavailability for storing and retrieving heat of the amount of molten salts needed to maintain the minimum liquid level required.
Typically, a minimum level in the range of 0.5 meters to 1.4 meters must be permanently left in a tank. Consequently, in the case of a 50 MW CSP plant with a heat storage system comprising two 38 meter-diameter tanks, it would be necessary the purchase of approximately 1,000 to 3,000 tons of nitrates salts mixture, which will not be available for heat transfer purpose.
Besides the burden on the investment, the storage of such an amount of unusable chemicals inevitably entails an undesired, very negative environmental impact due to the high consumption of energy and raw materials for their production, transportation and melting on an industrial scale.
The problem of minimizing the amount of liquid which cannot be sucked by a pump, namely a vertical pump, from a tank or vessel has been addressed with a variety of methods. With reference to FIG. 12, in one of this methods the bottom (B) of a tank (T) is provided with a sump area (SA) which acts as a priming reservoir of the pump (P). However, this known method cannot be applied to tanks working at high temperatures, such as the molten salts tanks employed in CSP plants, due to several disadvantages and difficulties to combine the thermal expansion which affect the metallic walls and bottom of the tank with insulation material of foundation which is required to avoid the concrete to reach a too high temperature.
The use of a tank having a non-flat bottom, as an example a bottom shaped so as to define a sump area, must in principle be avoided in order to allow, without any constraint, the radial expansion of the tank bottom when the tank is heated up to a temperature above about 180° C.
An objective of the present invention is to overcome the above-mentioned drawbacks of the known art.
Particularly, it is a scope of the present invention to provide an improved tank for liquids in which it is minimized the unusable amount of liquid that must be present in the tank for ensuring proper operation of an aspiration pump.
It is also a scope of the present invention to provide an improved tank for liquids which allows to maximize the amount of liquid which can be extracted therefrom by means of an aspiration pump while ensuring the proper operation of the latter.
In particular, it is a scope of the present invention to minimize the amount of molten salts (liquid) unusable as heat transfer and storage medium while assuring the proper operation of the aspiration pump used to suck them from the tank.
A further scope of the present invention is also the reduction of the environmental impact entailed by the production, melting, loading, storage and disposal of large amounts of chemicals such as molten salts.